They're focused on businesses and most would likely replace all their phones at a regular interval that's still too slow to keep them supported by security updates.
Android 10+ is what you support if you don't care about support for blatantly insecure devices without patches.
Any remotely competent IT department would make sure their devices are receiving security patches. Even a poorly run one would still be regularly replacing laptops and phones.
For personal devices, median income of people using Slack is definitely far higher than overall.