To answer the second part of your question first if you're not instantly sending a link, and reaching out to them one on one instead of copy and paste messages I think this is fine. I personally do this a lot in a non evasive way. I look for those who's content I think matches the site and I'll message them asking if they'd be interested in a site similar to x y z that they do. Then go from there if they respond.
Back to the first question:
GOOD & BAD TACTICS:
Do not DM Promote by sending copy and paste messages OR outright sending links. It doesn't matter where you are, this is probably against that sites ToS/Rules/ToU/etc.
Going places for the sole purpose to promote and not giving anything back. Hardly anyone will check out your service if all you do is promote your content and nothing else. People notice you subconsciously and if you're the guy plastering stuff everywhere and not talking versus if you're giving helpful, friendly advice to people constantly and occasionally link your site people notice the difference and will be more inclined to help out.
Not reading rules when you promote. Some places have more strict rules than others, some do not allow signature promotions, others do, don't just assume everywhere allows it, even if others do it. They could have special permission to do so, etc.
Just posting links, or links without a lot of context. It doesn't matter if you're on twitter or discord, people more than likely won't check out your link without context. "Check this out: domain.com" versus "Hey everyone, there's a new #domain website that is geared to giving you domain names at a fraction of the cost" This is a lot more appealing to those that may be searching from domain.