You’ll want to consider a vendor other than Apple then, as you want older support and older compatibility.
Windows is a good choice for your preferred usage there, as Microsoft has a history of trying to keep the older apps and older peripheral hardware working for longer.
Apple tends to be updating what is available to newer and that while also ending support for the older features earlier than some other vendors, and has a long history of that. Apple removed floppies earlier than most other vendors, and removed CD/DVD media earlier than many, and to complaining. Apple added other hardware and software features earlier than many other vendors, as well. HEIF/HEIC/HEVC, obviously. Going fully 64-bit for APIs and apps, too.
Here’s the first hit in DuckDuckGo for HIEC to JPG conversion, and there are other apps available:
https://www.microsoft.com/en-us/p/heic-to-jpeg-free/9ntvcmpjm5v3
Windows 11 is now in beta, and reportedly including hardware requirements past a number of existing computers so replacement computers will be getting newer birs, any discussions of longer-term hardware compatibility aside. Wouldn't surprise to see HEIF/HEIC/HEVC integrated with Windows 11 from the onset, and HEIF/HEIC/HEVC support has been present in Windows 10 since the October 2018 update; for most of three years now.
