Taxon swap drafted here : https://www.inaturalist.org/taxon_changes/140922

Top identifiers : @len-n-kayley @tiggrx @azik @littleowllondon @wnyjw @sedgequeen @radimjv @bobwardell @lern @charlie @duch : do you agree ?

My understanding is coming from the FNA (authored by George Argus). In the S. babylonica article, it's explained that S. x sepulcralis = S. alba x S. babylonica,
while S. x pendulina = S. babylonica x S. euxina. You can find more detail in the Flora:
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200005760

Here is an excerpt from I. Belyaeva's article of 2021, where she established the new combination and provided synonyms for it:
Salix × pendulina Wender. nothof. tristis (Gaudin) I.V.Belyaeva, comb. nov.
(= S. babylonica L. × S. × fragilis L. f. vitellina (L.) I.V.Belyaeva)
(urn:lsid:ipni.org:names: 77219497-1).
Basionym: S. alba f. tristis Gaudin, Fl. Helv. 6: 206. 1830.
19
≡ S. alba var. vitellina-tristis Ser., Essai Saules Suisse 83. 1815, syn. nov. ≡ S. alba var. tristis
(Gaudin) Trautv., Fl. Altaic. (C.F. Ledebour) 4: 256. 1833 ≡ S. × sepulcralis Simonkai

However, I don't think S. x sepulcralis should be listed a synonym of this new combination, as this is a hybrid of S. babylonica with S. x fragilis (the latter, in turn, is considered a hybrid of S. alba and S. euxina) rather than with S. euxina itself.

Although I can't find any date, I think the FNA entry (authored by George Argus) is older than Belyaeva's article of 2021

Here my advice: if S. x sepulcralis is no good anymore, you can swap it to its explicit combination : S. alba x S. babylonica, Also use the explicit combination for S. babylonica x (x S. fragilis). No one would argue that these are two different things and everyone would be clear about what each one is.

t_e_d
Exactly, that's my point: it's not, and yet the name S. x sepulcralis, which is alba x babylonyca ended up as a synonym of this name

The nomenclature of weeping willows is completely wrong at POWO as well as here at iNaturalist. We are now going to publish our taxonomic survey based on genome-wide sequencing data soon. I do not want to mention our results prior its publication, but Salix x sepulcralis is later synonym and should not be used any longer (I have checked the type specimen collected by Lajos Simonkai in Budapest). The taxonomic concept of I. Belyaeva (which is adopted by the POWO) is based on erroneous hypothesis (see e.g. here: https://doi.org/10.1002/tax.12685 or here: https://doi.org/10.3390/plants13050639). However, this concept of POWO (which lacks any support in DNA data) is unfortunately adopted on webpages world-wide (in contrary to scientific publications). I hope that POWO (and iNaturalist) will accept concepts supported by DNA data sometimes in future :).

I can't attest to DNA and all but in the UK the two are seen as distinct hybrids with different parents as azik describes. BSBI's Atlas lists them as:

Salix x sepulcralis = Salix babylonica x alba - https://plantatlas2020.org/atlas/2cd4p9h.pc2
Salix x pendulina = Salix babylonica x alba x euxina - https://plantatlas2020.org/atlas/2cd4p9h.95yzsy

Stace's New Flora makes the same distinction - however, my copy is an older copy that has Salix x sepulcralis as the above and then Salix x pendulina listed as S. babylonica x fragilis - but given that "S. fragilis" got split into S. alba x euxina around then, I'd bet the new one lists it as the hybrid of three. Not that I have it to check!