SO - I received my GDPR data and did some calculations. It seems that my unban date may have been set later than I expect. Let me attempt to explain how I calculated this:
1. The file that starts with activity-log: it contains a column called "Number Mins Banned." It seems that, for a permanent ban, this is 6311520, which is in minutes. Divide 6311520 by 60 mins per hour, and then 24 hours per day. This gives a figure of a 4,383 day ban - or 12.008 years. Most Habbo info on the internet normally lists a permanent ban as 11 years, but time-wise, it seems to calculate to 12 years and 3 days, assuming each year is exactly 365 days (not including leap years).
2. The file that starts with <account name>-<random numbers>-basic-data: the "Last Hotel Access" column should technically be the date that you were banned, as common sense will tell that you wouldn't be able to access the game after your ban date. In the Excel file, add +4383 to that date, and your technical unban date should appear. For me, this is a date in May 2019, and not the original September date that I had noted down for whatever reason.
If my calculations are correct, my ban may still be ending - just later than I thought. I may be completely incorrect, but the steps that I took are logical, and it would make sense if this works out. Either way, interesting to note!