I realize this is a bit esoteric, but if you ever find yourself needing to migrate messages from Slack to Discord, I can highly recommend slack-to-discord, a Python-based project on GitHub that I used yesterday to migrate some messages from Incomparable’s members-only Slack group to a new version of the community on Discord.
Free Slack instances only show 90 days of messages, but if you download your Slack archive—which is required to use this tool—you get everything. I was able to rescue a popular, long-running thread that originated five years ago. The utility replicates threads, displays the name and icon of each poster, and even displays emoji reactions and date- and time-stamps.
There are a few drawbacks. It’ll take a very long time to completely import a large Slack group, even at a rate of dozens of messages a minute. To mitigate this, you can limit imports to specific channels and date ranges. (With the exception of the 1000-post thread created five years ago, I mostly imported the last 14 days of messages into a bunch of channels, just to prime the conversational pump and make people feel at home.)
When I heard about this tool I was skeptical, but it vastly exceeded my expectations.