A few years after the European Tour allowed players to wear shorts in practice rounds, the PGA of America followed suit for its championships. Then the PGA Tour gave the OK to wearing shorts in practice rounds and pro-ams in February 2019. Now the R&A is following suit, allowing players in the British Open Championship to wear shorts in pre-tournament practice rounds.
That's great and all, but it seems unlikely many players will take up the R&A's offer. The British Open Championship has a rota of courses situated on links land, close to the coast, meaning it's susceptible to cool ocean breezes (or gusts and gales). Typically, players in what is now the fourth major of the year are wearing sweaters and jackets and beanies (or toques) as they compete in the game's oldest championship. Not shorts. But at least it is now an option for practice rounds, just in case the weather forecast turns a little warmer than anticipated.
With the Open being played at Royal Portrush in Northern Ireland this year, the expected July temperatures there are in the 60s. Perhaps some brave souls will be enticed into wearing shorts on a lark. At Royal St. George's in 2020, however, some English heat could force some players to show some leg.
Frankly, players should be allowed to wear shorts during tournament rounds pretty much anywhere around the world. High-level amateur and collegiate players do it, and they don't look any less professional or athletic by wearing shorts. The LPGA allows players to wear shorts year-round and in competition, and that doesn't seem to be a problem. Just cut the formality.