Glad to see you back on the forum OldSteve!
The jump from 1.6.x to 1.8.x happened after arduino.cc and arduino.org were merged back together. arduino.org had been using the 1.7.x versions for their IDE so .cc skipped right over 1.7 to 1.8. That's why wolvel is finding the Boards Manager feature missing from 1.7.10 even though it has been in the .cc IDE since somewhere around 1.6.4. Even though the 1.7 IDE has a higher version number actually it was based on an old 1.6 version and not significantly improved from that point.
Probably the biggest change that has happened since the 1.6.9 release is an upgrade of the Arduino AVR Boards toolchain. That added a higher level of compiler optimization (LTO) which can significantly reduce your sketch size in some cases. There was some growing pains, but that was actually related to the AVRDUDE version upgrade rather than the compiler and that was resolved in 1.6.11. The main problems I've seen people encountering with the new compiler version are limited to macOS.
There have also been a bunch of minor bug fixes and enhancements but that might not matter to you if you're not having any problems with 1.6.9. There have also been some new bugs introduced along the way but 1.8.5 has been pretty good. I can't think of any serious new problems with it.
They're actually working on 1.9.0 right now. That will have some significant changes, including a reworking of the sketch preprocessor. You may remember what happened the last time they did that in 1.6.6 but hopefully this time around it won't be so painful. They're trying to get beta testers for that version but I don't know how many people are actually doing it. You can find the download for it in the beta section of the Software download page. There have been some bug reports on the beta but it doesn't look too bad so far.