Working on my UI, and currentinly, in my interface, there is only the option to have 5 action bars, instead of 8?
Yes. Use an addon if you want more.
Use bartender addon on curseforge. You can have up to 10 action bars with it.
Does anyone need eight action bars?
Which ones have gone missing, and in which version of Classic?
Probably not, but I like using them for sorting purposes.