[Baertram]

Some addons provide purge settings, some history may day settings, other do not provide such settings.
In the latter case you can logout, backup your SavedVariables folder and delete the files of these addons in it to rebuild them.

Attention: this will reset other non history settings too in most cases.
If you do not want to keep guild store sell data older than 10 days as they are outdated and of no value for your sold items data you can setup addons like MasterMerchant etc. (using LibHistoire) to only keep that time frame, or delete the LibHistoire file (which will rescan all and might take some time and manually pressing the E keybind at the guild store sell history (to fetch more data).

Same for other big SV files. Check your folder if you still load these addons and if you do try their ingame settings to strip old chars/data or delete the SV files (while logged out!) and let them rebuild them.

Btw: Chat addons and any other addon load each SV file as you login, plus they will write them to disk from ingame memory at each reloadui or loading screen of a zone switch. This will make loading slow if you got addons enabled with the large SV data tables.
You should disable all addons you do not need while playing and only enable them as you need them. Create profiles with AddonSelector for example and use them for normal questing (disable crafting addons, and guild addons, sales addons). And my website end addons for dungeons and combat as you play them. This will increase game loading times a lot. I personally only use like 10 addons during normal game play and switch within 20 seconds between the profiles if I need it (20 seconds with a Ssd instead of a normal hard drive, and about 100 addons and libs active then, including LibHistoire for 5 guilds (but only 2 real sell tracked guilds, so with 5 of them loading time could increase due to bigger LibHistoire and TTC and MM data files).