This is a retroactive AITA as this was about 2 years ago. As such, I may not remember all the details. 

So 2 years ago I was in the midst of running a Pathfinder campaign (think D&amp;D) that was going great. Good, fun group who showed up regularly and brought good personality to the table. During April, one of my players ended up missing game for 3 weeks straight due to her working on her Master's Thesis. I recognize that as an important, and legitimate reason for missing the game. 

That being said, after the 3rd week, I told her that if she didn't start showing up to game in the coming weeks, I would not hold her spot in the game anymore. I only run 4-5 players at most and we had 5. Now, this was not a threat, nor was I kicking her out. It simply meant that if she didn't start coming regularly then I would give her spot to another player if one wanted to join and showed up regularly. I felt it was an imposition on me the GM to hold a spot for someone who wasn't showing up regularly. This situation was also made worse by some inter-player drama that occurred a month earlier, but that is irrelevant to this situation. 

Now, I admit in hindsight that I was a bit of a dick in the way I presented that to her, making it sound like an ultimatum. It wasn't, nor was it intended to be. While I could have handled the situation better, I feel my reasoning in the matter was sound. I wasn't trying to be rude or threatening, just informing her that after a certain point of missing the game that I felt I couldn't hold someone's spot. She eventually returned and all was great, but if you're going to be gone for a prolonged period, it's a bit of an imposition to refuse a new player for someone who isn't there.