There is nothing wrong with using tables in HTML5 or HTML4, except that they should be used correctly. This means, they should be used as tabular tables, not layout grids. Early web designers used tables for user interface layouts because CSS support across browsers used to be terrible. table layouts are much less common nowadays, but you'll still see them used all over the web.
Because UBB.threads base code was released in 2005 (and written prior), the base templates were based on table grids of that time. Around 2017 I took on the long task of upgrading all of the templates from HTML static grid tables and inlined styles, to a responsive layout capable of displaying on desktop and mobile screens, and moving most all inlined style to their proper CSS files. This work has maintained backward compatibility with UBB.threads from 2005, while functioning correctly on all screen sizes in today's world (2022). Current versions of UBB.threads are using a responsive system based on table grids and modern CSS standards.
Be aware that modern internet tech is still stuck using tables for email layout, if you need to support the majority of email clients unfortunately.
https://www.caniemail.com/features/css-display-grid/Also be aware that older browsers (Internet Explorer 11 and older) do not support CSS3.
For example, flexbox and object-fit only became available as of Edge 16, iOS 8.1, Android 4.3.
https://caniuse.com/flexboxhttps://caniuse.com/object-fitIn 2017 you stated, "Some of our members are using 2400 baud modems."
https://www.ubbcentral.com/forums/ubbthreads.php/topics/260341/restricting-imagesDo you still have users connecting to your modern website using 30+ year old ancient dial-up (POTS) hardware?
https://en.wikipedia.org/wiki/Modem#Evolution_of_dial-up_speeds