There isn't a one click "subscribe all" option.
You can set your forums to be auto-subscription by default though.
The options for the subscription mode are:
Optional - not currently subscribed but can opt in
Auto - subscribed initially, but can opt out
Forced - subscribed whether you want it or not
Disabled - not subscribed (and never will have the option to be)
You can also go the forum index page in any course which gives a nice summary of all the forums and allows subscription and digest changes - eg. https://yourmoodle/mod/forum/index.php?id=2 where 2 is the COURSE id.
You can set your forums to be auto-subscription by default though.
The options for the subscription mode are:
Optional - not currently subscribed but can opt in
Auto - subscribed initially, but can opt out
Forced - subscribed whether you want it or not
Disabled - not subscribed (and never will have the option to be)
You can also go the forum index page in any course which gives a nice summary of all the forums and allows subscription and digest changes - eg. https://yourmoodle/mod/forum/index.php?id=2 where 2 is the COURSE id.