Dell spec did not mention i9-10900.  However, in the 1st bios 1.0.9, 1.0.9 it mentioned "Fixed the issue where the system shuts down automatically when the CPU temperature reaches 40C. This issue occurs on systems with Intel i9-10900, i7-10700, i5-10600, i5-10500, and i5-10400 processors." this is a new model so there is not yet user benchmark validation.  what I am speculating is that the 3000 series model in Dell is usually entry level low tier, thus the top i9 cpu is not validated or offered by Dell.  however that is not equal to saying the i9 is not compatible with the 3891 chipset and bios.  there is just not evidence yet.  if I have to guess i would say chances are it is compatible.  Why do I think so?  B560 chipset (which is what 3891 has) has native support for i9.  Intel® B560 Chipset so I think it is very likely i9 is compatible with 3891.  again I wish I had user or vendor evidence to back up this claim. PS an analogy is Dell optiplex 3080 vs 7080.  Dell does not validate/offer i9 for 3080 because it is low tier, but does offer i9 for the high tier 7080.  Vendor has verified that i9 works in 3080.
I was in the market of upgrading, And i wondered, would it be possible to install an i9 cpu inside the dell inspiron 3891
CPU Upgrade Inspiron 3891