MC182488 announced the change to All plans back in June 2019, and that going forward it would not be showing groups that did not already contain plans. We have had a few support calls coming in where customers still aren't sure what changed here so I thought it was worth a little more explanation.
Before the change under All plans we would really show All groups. This wasn't too obvious - because if you clicked on a group that didn't already have a plan we would then create one for you. So it 'looked' like the group already had a plan. Now in some cases that was fine - but we heard that many of these plans were not really needed - and with groups becoming used for more scenarios across Microsoft 365 this meant more chance of accidentally creating a plan where one wasn't needed. To be clear, we never automatically created plans for new groups - you had to click on the group in All plans - but I can see how it may have looked like this based on the behavior described above.
All plans now just shows all plans - and you can also tell from the tile if the group probably contains multiple plans - as the plan name (bold) doesn't match the group name show below it.
You can of course create a new plan and choose and existing group to add it to - but we avoid the clutter and potential problems that showing all groups was causing.