Thanks for the info Joe. I noticed that the website was down and I wondered about the app. It is sad that its disappearance was so little noticed.
The category of commercial word processing seems to have been decimated by text processors for most uses, by free apps like Pages on the low end, and by Microsoft for the bloated do-everything high end that corporate sales support.
Most people never needed feature rich, increasingly complicated, ribbon-iconed processors instead of basic frictionless writing tools. That’s something even Apple hasn’t figured out, given the slightly slow, clunky implementation of Pages in iCloud, and the app’s included layout functions that are increasingly irrelevant to modern use (Business card template? Drop-in iMovie functionality?)
If Nisus does start glitching with new macOS versions, it will be hard to find another word processing app with a style-aware regex engine. Maybe you could hack something with Word’s VBA macros - but even then the Mac version of Word has bowlderized functionality and lacks VBScript/COM, I believe. On Mac I suspect you’d need to complement a replacement word processor with something like BBEdit which supports regex and style natively.