This dialog comes from the System Management framework [1]. Slack is probably installing a privileged helper tool (conceptually similar to a setuid root binary) so that it can update itself regardless of where it is installed or which user originally installed it.
Seems like it should only need to do this once. I get this with almost every Slack and VSCode update. The correct solution for me is to quit Slack.app and let my company's management software do the update for me.
Chances are they have some kind of management software like SentinelOne that is preventing Slack from doing this (or storing the permission to do so), so it just asks over and over. Which is arguably worse.
Discord does this as well I believe. I often needed to enter the administrator password to install a helper after the system had been off for a couple days.
A software updater was going to be my best guess at what this was. I guess I understand the flexibility it brings, but it definitely does have some security trade-offs.
[1]: https://developer.apple.com/documentation/servicemanagement/...