Hi,
we're also on 8.8.15 (latest patch minus 1), on CentOS7.
I've basically just migrated everything to S3 backup and HSM because I - foolishly one might say - assumed it would be much easier to do migrations once you don't have 2TB of data sitting in a VMWare volume. The documentation even pointed that out (in some way)...
Maybe an option would be to EOL Zimbra 8 on platforms that are EOL anyway (CentOS 6+7, old Ubuntu releases) and then continue to build 8.8.15 patch releases only on recent OSs.
Of course, we don't know how difficult it is to maintain 8.8 and where the "pain-points" with it are - AFAIK, it has never been communicated. From cursory looks, it seems a lot of components were updated over the years (MariaDB being an exception).
I admit I haven't spent too much time on upgrading even to 9. I have other duties and I just "chase" the patch-releases - it's planned for Summer, though.
If you (Zimbra) think you can figure out something in the course of the year, that's good enough. My plan was to upgrade to 9 through Summer/Autumn and then I hope there is a sensible path to 10 without undoing all the S3-stuff I have (and which took a rather long time to be actually usable).
I don't really want to put any more pressure on you guys, but please: Help us