Is there a comprehensive list of all sports activities and which Garmin supports which?
This list appears elusive and evaded my searches. Many thanks for considering my question. 
I love my fenix 6x Sapphire. So I bought my son the Instinct Surf edition. But it doesn't support bouldering and climbing. Is this true?
I want to avoid the mistake if I gift other Gain watches to friends and family. Cheers.
Not a comprehensive matrix, but might be what you're looking for. From the main Garmin site, wearables, all wearables & smartwatches. Then choose the activity you're interested in.
But it doesn't support bouldering and climbing. Is this true?
Yes, this is true. To confirm you can use the compare function to compare a list of 2 to 4 different of watches, for example fenix 6X vs Instinct Surf
