I'm bootstrapping, How to get free credits for Notion,GitHub,Slack etc.?

Not sure it's worth the time looking for free credits. Even if you get a year for free you'll end up saving maybe $500 max.

If you're worried about costs, look for alternatives. In the apps you've listed, only Figma is a real useful app.

Notion is just documents, a markdown editor and a shared drive will give you 99% of Notion you need. The rest is overengineered anyways.

Github free tier gets you pretty far already.

Slack is just chat, tons of free alternatives there.

not sure about those you mentioned, but yc startup deals can give you other things.

go to startupschool.org

create your company profile

post weekly updates for 2 to 3 months

apply to yc startup deals, get $5k aws credits, discounted stripe, etc.

apply to next yc batch.

As @d--b mentioned, look for alternatives.

I use Gitlab free tier (I can even use the CI/CD as I have a Linode small VPS box - US$5/month with their runner installed - no minutes from their end needed).

Rocket.Chat used to be a very look-a-like Slack.