My freelance contract is about to end. There are work chats to collaborate with other companies(API providers) which participated tests several times with me. Is it fine to leave a message similar to "I'm leaving this chat since my contract is done. Thank you for your help." before I leave, so I can hint others that I'm available for another work? Or should I leave quietly without bothering them?
Don't hint. These are contacts who know you and should be inclined to think well of your expertise. Contact them separately outside the company if you feel it will be useful and let them know you're available for work.
Generally one should avoid using a companies infrastructure to solicit work and the best results are targeted ones. You being available for future projects will be irrelevant to some and an individual contact is both more personal and more valuable as it's a bit harder to just ignore.
Agree with post from Kilisi. Anyone who knows you and has any interest in working with you again, they're capable of finding you on LinkedIn or through mutual contacts. If you were to say nothing other than "goodbye, wish you all the best, etc.", then you leave room for those people to reach out to you.
On the other hand, if you try hinting, nudging, etc., that will:
- not make any difference whatsoever for people already uninterested in working with you again; anyone who was on the fence isn't going to be "won over" like this
- might annoy people - the "annoy people" outcome is what you want to avoid
Just be polite to everyone, say your goodbyes, and leave it up to them to reach out. If nobody reaches out, you can at least be sure you didn't annoy anyone along the way.