Instead of using my MacBook with an external monitor, I decided to revive an old 27" iMac by upgrading its HDD to SSD, which meant reverting to High Sierra. When the upgrade to 8,8 was advertised, I went to download it, but found the instruction that if using High Sierra, version 8,5 should be used. I already have 8.6 on the machine, which seems to work.
Sorry if this seems a silly question to ask, but what is likely to happen if I carry on using 8.6? In other words, is it important to downgrade to 8.5 on High Sierra. I can still run an up-to-date version of TB on my MacBook or in a Parallels VM, but it would be convenient to run it on the iMac. Of course, the best option would be to buy a new iMac, but that is a bigger decision.