Fanaelialae
Legend
Now get me that list of easy methods for reducing the speed of adult dragons to zero, so they fall out of the sky without being in melee combat requiring fly.
I can give you two, though there are undoubtedly more (I only checked up to 5th level wizard spells).
Bigby's Grasping Hand and Telekinesis. Which are spells that you probably have access to by the time you're facing adult dragons.
Bigby's is is a decent default choice. It uses a +8 ability check which matches or beats that of the majority of dragons. As a bonus, you can do a bit of damage to the dragon. It's unclear whether the grapple would cause the dragon to fall outright or if the hand holds it in the air and has to drag it down (which might take a few rounds depending on how high it's flying). Since the dragon can try to escape every turn, this is arguably the lesser option unless the DM rules that the grapple causes the dragon to fall from the sky automatically. So it will vary campaign to campaign.
Telekinesis is a great choice if you have a bit of backup. Someone in this thread mentioned that they had a cleric, bard and wizard in the party, and this is the perfect setup for Telekinesis. With the bard's inspiring word and the cleric's guidance, your Telekinesis has a good chance to beat even ancient dragons in an ability check (wizard's +5+1d10+1d4 = +13 vs dragon's +8). The really great thing about this spell is that it restrains the dragon until the end of your next turn (dragon has no chance to escape before then). So your allies have advantage on their attacks!
Note that since both of these spells utilize ability checks, legendary resistance cannot help the dragon in question. That's the key.
There are also other, non-magical options. For example, anyone with the Sentinel feat just needs to be strafed by the dragon to lock it into position for a melee beat down (or simply get adjacent to it before it gets airborne, perhaps with the help of a mage's dimension door spell).
Against the Young varieties it becomes even easier, especially since they lack legendary resistance. A battlemaster with a bow can trip attack them out of the sky, or a caster could use the EE Earthbind spell.