https://twitter.com/mikemearls/status/1055927779336761344Few people asked, latest version of two weapon fighting:
* Does not use bonus action, off-hand gets full damage bonus
* Choose one of the following when you attack:
1. Get one extra attack with off-hand, but all attacks at disadvantage
2. Add your off-hand damage die to all atks
If you have the extra attack feature. disadvantage applies only to off-hand. If that feature gives you another attack (3 total base), you no longer have disadvantage on the off hand.
This nerfs TWF for the one edge case where the old TWF rules were favourable (pure-classed Rogues wanting a second chance to apply their Sneak Attack in case that the first attack misses), except that their bonus action is now freed up for other stuff.
OTOH, for anyone else option #2 doesn't seem too terrible. 1d8+1d6+mod (e.g. longsword+shortsword) is slightly ahead of 2d6+mod for THF without feats or fighting styles, and the off-hand die adds up with extra attacks.
Not sure if there ever is a point where #1 is preferable. Disadvantage on all attacks is terrible, even if you get two of them early. 2x 1d8+mod and an extra 1d6+mod attack at disadvantage is terrible at Extra Attack(1). 3x 1d8+mod + 1x 1d6+mod at Extra Attack (2) is straight worse than 3x 1d8+1d6+mod, and more extra attacks just skew the numbers even more in favour of taking option #2.