3 times in 10 years at the Proms, the last time being 5 years ago
I'm a little confused by this – there's been contemporary works at the Proms with far, far greater frequency than what you've just cited.
Looking at 2021:
30.7 –
When Soft Voices Die (premiere) by Sir James MacMillan with Dalia Stasevska and BBCSO
2.8 –
Cloudline (premiere) by Elizabeth Ogonek with Ryan Bancroft and BBCNOW
5.8 –
The Exterminating Angel Symphony (London premiere) by Thomas Adès' with Mirga Gražinytė-Tyla and CBSO
13.8 –
Spring's Shining Wake by Anthony Payne with Martin Brabbins and BBCSO
15.8 –
Qhawe,
As You Are, and
Lerato by Abel Selaocoe with Clark Rundell and BBCNOW
15.8 –
Bambara and
Dounia Iafou by Simo Lagnawi (same concert)
19.8 –
Many Are the Wonders by Ken Burton with the BBC Singers
19.8 –
Birdchant (premiere) by Bernard Hughes (same concert)
19.8 –
A New Flame (premiere) by Nico Muhly (same concert)
19.8 –
Ave Verum Corpus Re-imagined by Roderick Williams (same concert)
19.8 –
Aetherworld (premiere) by Shiva Feshareki (same concert)
23.8 – Several works by Ariel Ramírez with Sean Shibe and Adam Walker
27.8 –
Where Icebergs Dance Away by Charlotte Bray with Sakari Oramo and BBCSO
31.8 –
The Way to Castle Yonder by Oliver Knussen with Sir George Benjamin and Mahler Chamber Orchestra
31.8 –
Concerto for Orchestra (premiere) by Sir George Benjamin (same concert)
5.9 –
The Imagined Forest (premiere) by Grace-Evangeline Mason with Domingo Hindoyan and RLPO
7.9 –
Subito con forza (UK premiere) by Unsuk Chin with Sir Mark Elder and Hallé
Source:
BBC Proms
Note, too, that I'm mainly focusing on music written within the last 50 years. If I were to include everything written after 1900, the list would be much, much longer.