First, one thing to clear up, according to the Wikipedia article on Time control, Bronstein Delay is not exactly "time delay" from the Laws of Chess. With "time delay", the player's clock does not start ticking down until after a period of thinking time has elapsed. In Bronstein time, the clock ticks down immediately, so you can lose on time (as far as I'm aware), but time is added back to your clock after your move as in incremental timing except that you won't get more time added than you spent thinking (so if you thought for 7 seconds in a game with 15 second increments, you will only get 7 seconds added back to the clock, but if you thought for 30 seconds, you will get 15 seconds added back). The net effect is equivalent to the simple delay, but I believe Bronstein allows you to lose on time in situations where a simple delay would not. Actually, upon further reading, the difference between Bronstein and simple delay is purely in how the clock presents the total remaining time as the delay is added at the beginning of the game on turn one, so a loss in time would occur after the same total thinking time for both time controls.
Regardless, that isn't quite what your question is about. So let's move to the PGN specification. As best as I can tell, the PGN specification does not provide any standard way for notating a delay or a Bronstein increment. Of particular note is this answer to a similar question from about 7 years ago.
The only suggestion is the standard one regarding these questions about PGN which is to put the information in a comment field. The standard is old and no longer maintained as far as I know.
I would agree. The TimeControl tag is optional, so you could omit it altogether in favor of an explanatory comment, or you could add a value that seems reasonable and then a comment to explain.
[TimeControl "?"] ; Bronstein, 30 min, 15 sec delay
[TimeControl "1800+15"] ; Bronstein
[TimeControl "1800"] ; 15 second time delay
Regardless, if you import this sort of notation into any parsing software, it would likely not show you any of your comments around the TimeControl tag, so depending on what you want to do with the PGN file, it may be irrelevant.