I’m in two places here, my personal thoughts and MT thoughts.
My personal thought is that there is only so much SOTA jam to spread about and the more sites the thinner that jam gets spread UNLESS you take measures to ensure the same jam gets spread everywhere.
If the idea is to use a bot running somewhere to use the SOTAwatch API to get the spots and paste them on a Slack channel, then there is no issue. Some seconds after a spot appears on SOTAwatch it will appear on Slack and the users get a popup. You can get popups from some other apps already. If you intend to spot onto Slack and have those spots forwarded to SOTawatch then again there is no issue. Users can use Slack as an alternate front end to the spotting engine. The jam is spread at the same thickness everywhere.
I’m sure for that Christophe ON6ZQ’s filtered spots page can cause popups (using WebRTC I assume). That may be a solution that is simple to use. There was a Twitter feed of spots that Martyn M1MAJ ran. Now Martyn has taken early reitrement (the swine) so he may not have access to that server anymore. It’s not any real effort in that case for me to resurect my Twitter feed of spots. I turned it off because having learned how to send tweets, nobody was using it so it waste of bandwidth and electricity. Either may suit your needs without having to do anything else.
My MT thoughts are simply protecting the viability of the services I help to run. If you want to use the exisiting feeds to place spots into your Slack channel, I’d want to know you will not be doing anything daft that puts an undue load on SOTAwatch. Otherwise you’re free to go.