I empathize with many of the complaints, but some are a bit ridiculous. Using custom fonts in software UI is completely reasonable and normal, even if you don't like it.
You'd get Office 2003 and it'd follow the Windows XP style with lots of blue [1] and you'd get Office 2004 for Mac with the brushed metal styling [2] - and many applications only targeted a single platform.
Whereas in the modern age you get Slack for Web, Slack for Windows, Slack for Mac, Slack for Linux, Slack for iOS and Slack for Android - and it tries to be consistent across different platforms, instead of being consistent with different platforms.
[1] https://en.wikipedia.org/wiki/Microsoft_Office_2003 [2] https://en.wikipedia.org/wiki/Microsoft_Office_2004_for_Mac
It was at win8 where everyone just noped out and just started doing whatever they wanted. XP/2000 was the last era where anyone really cared.