I have also received the https:// reminder email on 10/13/2023 from Team at Maxmind. I had updated to CSF 14.20 on 10/05/2023. I am guessing that the Maxmind folks used a logfile that reached before my upgrade date of 10/05/2023 to generate their reminder email.
I have taken a look at the lines 336 to 338 in /usr/local/csf/lib/ConfigServer/Config.pm and confirmed that the code had been updated to https://. Also, my csf.blocklists file has been updated to https:// by the upgrade to 14.20.
I think you can safely disregard the warning email from Maxmind if you are running 14.20 and have not somehow altered the abovementioned files with an older version copy.