Steam chat unable to load old messages now

not some lucky glitch, I also had chat log

I said "lucky", not "unique", and Steam is well known for having distinctly different (as well as those which are largely overlapping) quirks for individual users. Some of the glitches I've encountered have only been shared by 5~20% of the friends I've spoken to. Others, only 80%. Getting 100% coverage from a Steam bug actually seems to be rarer than not, barring the full on mayhem that has occured during certain major Steam sales.

And, again, yours is an experience that noone I've known has had (nor anything that any of the many websites I pulled up mentioned, with all of them being in line with my experiences both pre and post revamp) , and something that I recall Valve explicitly addressing when they brought up the revamp. That doesn't mean it wasn't possible- perhaps through an obscure setting or configuration edit that even Valve forgot about, or even that everyone I knew were the ones who were bugged. It just means that it was something an apparently quite notable number of us didn't have access to.

As covered already, mobile does apparently have a different setup, and it is apparently currently possible to reliably(?) access chat logs for up to two weeks there. Further googling also suggests that the pre-revamp mobile app used to reliably save the most recent 200 lines (same cap as pre-revamp PC client, but apparently the mobile version didn't delete the logs once client closed or after a pariod of time like client does), regardless of time spent between conversations, so that would suitably work to explain your and the OP's experiences, if there was any chance that either of you used mobile chat in the past.

Certainly, I never had those experiences of yours on PC (rather, they're arguably worse now, as now chat fully clears its logs whenever I close Steam, and instead of randomly wiping the oldest parts of chat after a few days, it now randomly wipes random parts of chat) and, again, I'm not finding any supporting sources on the matter (as it relates to your favorable environment having existing on PC), and instead am bumping into countless sources that support the idea that PC did wipe logs after a few days or after 200 lines, pre-revamp, and can also only find support for the idea that the PC version is currently glitched out of its gourd, without any exceptions.

If you were lucky enough to use mobile in the past or at present, your experiences apparently will have been much better. If you're going to assert a contradiction to the PC experience, then I can only take you at your word if I also assume it's "Steam being Steam" or some abberant circumstance, without some sort of supporting reference to consider- because, again, the first few search pages of every search I put in seem to be unanimous in supporting my experiences.

That doesn't mean those perspectives are absolute, or even that they were the dominant experience for Steam chat users (bad experiences do tend to get a biased visibility, after all). It just means they're absolutely the expectation that many of us had and have for Steam chat, and your experiences are a novel contradiction to that expectation that very much needs some kind of framing to it- even if that framing is just "Steam is glitchy and this is just Steam being Steam, and thereby you're lucky where we were not."

TL;DR version: Probably a lucky glitch circumstance of you using mobile software where I and those I know did not.. :P