Me too, as far as remembering, as the years advance. It was version 5 or one of the major release points, where things really started to "go backwards". Among other things, they changed the way the UI worked, in ways I found not just different but significantly less effective and more cumbersome. IIRC, some of this might have related to changes in the underlying libraries or versions of libraries they were using.
Around the turn of the century, I was using Textpad as part of processing some relatively massive text inputs. It remained solid and much more performant than the alternatives I tried. This including its regex support, which was great for examining, munging/transmogrifying, and cleaning up text data. Particularly text data containing significant variability and some one-off scenarios; having ongoing visual oversight and the ability to rapidly, manually select areas to affect in various ways, was vastly superior to trying to come up with command line / whole file invocations that would create the equivalent impact.
Additionally, other editors choked on the volume/size of data files I was dealing with.
What's that "open" alternative that's so popular? Notepad++, I think. Years later, I needed to "beat up" some text and tried it. Despite this being years later -- more time for development and refinement -- it was nowhere close to Textpad. And while it had a bazillion features and functions built in, I found those I tried to use very "shallow"; get beyond the simplest use case, and they started to bog down and "glitch" with unexpected and/or inconsistent behavior.
I remember trying to use it for some regex manipulations, and having to give up on that feature.
Finally, now that I recall, Textpad was "nagware". You could actually use it substantially and really get a feel for it. I paid for my license gladly.