Well, I guess that will be a methaphoric slap in Microsofts face: Microsoft 365, v2609 working on Windows 8.1 (not even updated): Funny enough, this version "offically" dropped Windows 10 BUT is installable on 8.1 with @abbodi1406 patch
I've had some installs stuck on Version 2608. This started either in mid-September when Version 2609 started, or early October with 20430.20146 (with a downgrade). Despite having CTR set to Current, they appear to be following Monthly / 2021 Retail. Other installs have continued to upgrade to build 20430.20182 (including .20180) and I suspect they will upgrade to 20527.20088 in coming hours. These might have C2R vNext licenses, but I am yet to confirm this. All were installed with ProPlus 2024 Retail and then converted to VL (C2R-R2V). This would usually limit the build to 17932.21056 (as it becomes 2024 LTSC starting at 17928.20148) - at least as of Oct 13 - so perhaps 20326.20232 isn't so bad...? If I had to guess, I'd say MS have perhaps changed entitlements. The affected C2R client version appears to be 16.0.20430.20146 from Oct 6.
Looks like my issue was the same. I had some Windows 11 PCs stuck reporting, but once resolved it was all Windows 10 and Server < 24H2 boxes remaining... Nice find. Looks like they might track monthly from now until October 2028...
@DrStrange - I received this Version and Build just today, a few hours ago, during a regular update, and there were no problems at all.