Blizzard announced that Overwatch is getting crossplay soon but that there will be a few caveats involved. There is also a Legendary in it for everyone.
Overwatch will get crossplay in near future but Blizzard didn't specify the date when we will be able to play with friends on other platforms. Other than that, the dev update revealed pretty much all the necessary information.
When crossplay hits, the game will prioritise matchmaking console players with other console players and similarly, PC players vs PC players. Should a party of friends be a mix of PC and console players, they will also be matched versus PC players or other mixed parties.
Furthermore, if console players match with friends on PC, their aim assist will be disabled even though they are playing on controllers.
Competitive will be an exception as PC players will not be able to party up with console players. They will remain separated here for balancing reasons, even if you have a controller plugged in on PC.
Players will be able to opt-out of crossplay but it will be enabled by default. Simply visiting options and changing the setting will allow you to choose to play only against the same platform.
Blizzard also reminded players that crossplay will be in beta when it launches, meaning some parts of the system may be missing or not functioning as intended. One of them is the cross-progression which will not be available at the crossplay launch.
It is probably for this reason that Blizzard will compensate the players with a Golden Lootbox. These are some of the most important details for the system but if you have more questions, it's possible Overwatch crossplay FAQ can help.