user91197: I am running lightning with the LND implementation and a full node. Nevertheless, I heard that it is possible to use Lighting in pruned mode.
How is this possible?

I read in the LND install documentation that you need the -txindex flag:


In order to run lnd with a bitcoind back-end, the bitcoind instance must be configured with --txindex just like btcd above.


In the Wikipage of Bitcoin it says for pruned mode: 


This mode is incompatible with -txindex

I also read here and here the same arguments.
Is this outdated or is it not possible to use prune mode in lightning?

user90088: You are correct in that you cannot run a LND node with a pruned bitcoind server. 
Pruning isn't compatible with transaction indexing because the index would point to transactions which have been pruned (no longer in store).
Each lightning node requires transaction indexing, because it will be looking for specific txid's on-chain throughout the lifetime of the payment channel.
Broadcast and confirmation of:

Funding transaction
Commitment transactions in a unilateral close
Closing transaction 

The only way to determine whether any of the transactions above have been confirmed, is to query an up-to-date transaction index of the strong chain, which is not possible with a pruned node.
Lightning nodes also subscribe to the ZeroMQ subscription service of the bitcoind node, but this only tells the node about newly broadcast transactions accepted by in the mempool, not whether they have been confirmed yet.

user91626: Update(2021): Since version 0.13 pruned nodes are officially supported although they cause performance penalty and higher network usage.
It is possible to run Lightning (both LND and c-lightning) with bitcoind pruned mode.
There are already packages turning the pruned mode on when disk space is limited: Lightning Power Node Launcher (works with LND) and BTCPayServer docker (works with c-lightnig).
There is a lot of conflicting information online, because it became possible only after Bitcoin Core 0.16.0 was released.
Update from the official lnd documentation:

We don't require --txindex when running with bitcoind but
activating the txindex will generally make lnd run faster.
NOTE: WE DO NOT FULLY SUPPORT PRUNED OPERATING MODES FOR FULL NODES.
It's possible to run a node in a pruned mode and have it serve lnd,
however one must take care to ensure that lnd has all blocks on disk
since the birth of the wallet, and the age of the earliest channels
(which were created around March 2018).


91197: <p>I am running lightning with the LND implementation and a full node. Nevertheless, I heard that it is possible to use <strong>Lighting in pruned mode.</strong></p>

<p>How is this possible?</p>

<ol>
<li>I read in the <a href="https://github.com/lightningnetwork/lnd/blob/8cce2ad6304cc0e1919eeac724ec1c21e952826e/docs/INSTALL.md#running-lnd-using-the-bitcoind-backend" rel="noreferrer">LND install documentation</a> that you need the -txindex flag:</li>
</ol>

<blockquote>
  <p>In order to run lnd with a bitcoind back-end, the bitcoind instance must be configured with --txindex just like btcd above.</p>
</blockquote>

<ol start="2">
<li>In the <a href="https://en.bitcoin.it/wiki/Running_Bitcoin#Command-line_arguments" rel="noreferrer">Wikipage of Bitcoin</a> it says for pruned mode: </li>
</ol>

<blockquote>
  <p>This mode is incompatible with -txindex</p>
</blockquote>

<p>I also read <a href="https://www.reddit.com/r/Bitcoin/comments/7rz2l5/prune_node_lightning_node/" rel="noreferrer">here</a> and <a href="https://bitcoin.stackexchange.com/questions/52889/bitcoin-core-txindex-vs-default-mode-vs-pruned-mode-in-depth">here</a> the same arguments.</p>

<p>Is this outdated or is it not possible to use prune mode in lightning?</p>
 90088: <p>You are correct in that you cannot run a LND node with a pruned <code>bitcoind</code> server. </p>

<p>Pruning isn't compatible with <code>transaction indexing</code> because the index would point to transactions which have been pruned (no longer in store).</p>

<p>Each lightning node requires <code>transaction indexing</code>, because it will be looking for specific txid's <code>on-chain</code> throughout the lifetime of the payment channel.</p>

<p>Broadcast and confirmation of:</p>

<ul>
<li>Funding transaction</li>
<li>Commitment transactions in a unilateral close</li>
<li>Closing transaction </li>
</ul>

<p>The only way to determine whether any of the transactions above have been confirmed, is to query an up-to-date transaction index of the strong chain, which is not possible with a pruned node.</p>

<p>Lightning nodes also subscribe to the ZeroMQ subscription service of the bitcoind node, but this only tells the node about newly broadcast transactions accepted by in the mempool, not whether they have been confirmed yet.</p>
 91626: <p><strong>Update(2021):</strong> Since version 0.13 pruned nodes are officially supported although they cause performance penalty and higher network usage.</p>
<p>It <strong>is possible</strong> to run Lightning (both LND and c-lightning) with <code>bitcoind</code> pruned mode.</p>
<p>There are already packages turning the pruned mode on when disk space is limited: <a href="https://github.com/PierreRochard/node-launcher" rel="nofollow noreferrer">Lightning Power Node Launcher</a> (works with LND) and <a href="https://github.com/btcpayserver/btcpayserver-docker" rel="nofollow noreferrer">BTCPayServer docker</a> (works with c-lightnig).</p>
<p>There is a lot of conflicting information online, because it became possible only after Bitcoin Core 0.16.0 was released.</p>
<p>Update from the official <a href="https://github.com/lightningnetwork/lnd/blob/master/docs/INSTALL.md#available-backend-operating-modes" rel="nofollow noreferrer"><code>lnd</code> documentation</a>:</p>
<blockquote>
<p>We don't require <code>--txindex</code> when running with <code>bitcoind</code> but
activating the <code>txindex</code> will generally make <code>lnd</code> run faster.</p>
<p><strong>NOTE: WE DO NOT FULLY SUPPORT PRUNED OPERATING MODES FOR FULL NODES.</strong>
It's possible to run a node in a pruned mode and have it serve lnd,
however one must take care to ensure that <code>lnd</code> has all blocks on disk
since the birth of the wallet, and the age of the earliest channels
(which were created around March 2018).</p>
</blockquote>
