Exactly what the topic says: I would like to filter monsters by specified abilities. Maybe the druid in the party wants to use Conjure Animals to summon multiple beasts to surround the opponent, so I'm nice and choose primarily beasts with Pack Tactics (if there are any at the chosen CR). Same scenario but the druid wants to track something, then I can look for beasts with Keen Hearing and Smell. Or maybe I want to create a random encounter based around some rare monster with Spellcasting, and so on.

Monster abilities and attributes are already a searchable characteristic from the main search bar, such as with this result for 'Pack Tactics'. I think there are likely too many different abilities used each by too few monsters to make it a filterable category in the monster view like tags or source. Although I agree that a general search option within the filter fields might be a useful addition.

 It wouldn't be hard, they just need to give us the option to filter by ability text.

If you think the current system works, try finding a list of spellcasters (with slots).

Quote from Davedamon >>

Monster abilities and attributes are already a searchable characteristic from the main search bar, such as with this result for 'Pack Tactics'. 

I didn't even think of trying this, but it feels like clicking on page 2 for your Google results because I have to limit the search to monsters after the search results are shown. I already know I'm searching for monsters, so indeed a general search term added to the filters would be better.