"I reassured him that version 2.1.x had no declared end-of-life for the time being, and that we could commit to 3 to 5 years of maintenance, but"
- This is not correct. EOS of 2.1.x is 30/04/2029.
Also, a 3.1 version will be generally available end of 2025, with TM1 server V12 (not V11), this is V12 without a containerized TM1 server version and hence no high availability support.