OPie: Re: WoW Classic Support
You can install and use recent versions of OPie on Classic WoW clients (pretty much all variants). It will probably not explode. You may need to check the "Load out of date AddOns" checkbox in the AddOn List dialog. Some OPie functionality may not be available. Please report any issues you encounter on Classic clients, but be aware that debugging non-trivial issues on Classic is not a priority.
Why?
OPie functionality tends to run into WoW client quirks that are annoyingly time-consuming to investigate and work around. For example, the WoW client sometimes decides that when you try to cast Travel Form, what you really meant to cast was Travel Form (Rank 2), which happens to be a passive ability, and so you have no business trying to cast it; have an error message for your efforts.
Fixing such issues for Classic may require levelling characters or professions; having to repeat that process across several game variants is too time-consuming to be personally worth it.