Legacy Extensions

If you upgrade to Mojave (10.14.x) or above any 32 bit applications will not run. You can download RoaringApps to find what are 32 bit apps or just open System Information (Applications - Utilities - System Information) and click Software - Apps and it will populate and show what are 32 bit apps and what are 64 bit apps. File extensions should not make a difference, what does is the app itself. For example, if you have Office for Mac 2007 installed, that is a 32 bit app and will not run on Mojave or later, however all the data files are fine. Just update to Office for Mac 2019 or get Office 365 and you are back in business.

No need to worry, but to see them, just don't upgrade to 10.16 if the developers don't have updates...

About this Mac>System Report>Software>Extensions, click on Obtained from Header, all that aren't Apple are Legacy.

This Apple article talks about it:

About legacy system extensions - Apple Support

but does not show how to check for them. This is a piece of what System Information, mentioned by DBAqua, shows when you select Extensions:

By far most are Apple extensions and we can assume that Apple will deal with them. Most of the non-Apple extension are ones Apple gets from outside partners, so I don't worry about those either.

Your welcome, however I see I made an error. 32 bit apps will run on Mojave, they will not run on Catalina and later.