Uh, about the Jeremy Crawford`s sage advice, I´m not sure but the sage advices are official material of WOTC so since they have D&D consultants in Larian helping with the game I assume that they already implemented that rule into the game (and some others). Happy to help a fellow crazy guy!
Warcaster is still a very useful feat, maybe still could have a place in your builds.
If they colour "melf´s acid arrow" in pink I´m sorry but I´m out...
Edited: If an arcane caster has a spellcasting focus, can they use that hand to perform the somatic component?
Yes, of course, you need a free hand, but you can use the same hand that holds the spellcasting focus to perform the somatic component. And wizards and Sorcerers can use a staff as spellcasting focus, for example.
Bards use his favourite instrument as a spellcasting focus (Some subclasses the weapon too), like drums, a Lyre, or a horn.
And if you have the warcaster feat you can you can perform the SOMATIC components of spells even when you have weapons or a shield in one or both hands. You still need a hand free to have access to your materials or your spellcasting focus if the spell requires MATERIAL.
But as stated, you can have a shield in one hand and a staff-spellcasting focus in the other hand and cast all spells (somatic and material) using that hand.