Quick question if we wanted to maintain same version for the api and don’t want to introduce more mandatory fields so we are backward compatible how do we introduce those fields ( make them optional ) ?

You mean, how do you specify optional fields (e.g. to let people know they exist, but won't break tests)?

Yes mainly in the request

In a consumer request you can specify extra fields that aren't load-bearing. It's up to the provider to care about if it needs them or not. Without that, you'd probably have trouble with the grow-then-shrink approach.

So we end up versioning or keeping the required field behind a feature flag ?

Might be easier to explain with a concrete example.

How about this - Add a new optional key - change the behavior based on this key present, retain the old behavior if the key doesn’t exist, and once all consumers start sending the key make it mandatory ( consumers should know once they start sending this eventually this will be made mandatory ) , if consumer wants to retain the old behavior then maintain this as optional

Might be easier to explain with a concrete example.

I agree with this point, it would be good to understand the example

Yep, that sounds right. Just be aware that making it mandatory is technically a breaking change. If you have tests for the case where it's missing, they should change, and/or be removed.