For an idea I'm working on, I compared the list of ISO 15924/Unicode scripts with what is currently displayed under either section of the script list in the release editor. There are a bunch of scripts missing, and one displayed which is only a proposal. Most or all of these might never see any use, but they're no less likely to be used than some of the others (e.g., Linear B is in the list, Linear A is currently missing).

I also tried seeding the release editor with an import using some of these script codes that are missing from the RE dropdown, in case it was simply a case of the scripts not being displayed, but that had the same result as not seeding any script - no script was set.

I'm not including here the script codes that aren't actually scripts (private use, inherited, etc). These are all actual defined scripts, they're just currently not accessible when adding or editing a release.

Khutsuri (Geok) is only a proposal. A quick search makes it appear that there are no releases in the database currently set to that script.

ISO 15924 code: Script name

Adlm: Adlam
Aghb: Caucasian Albanian
Ahom: Ahom, Tai Ahom
Armi: Imperial Aramaic
Avst: Avestan
Bamu: Bamum
Bass: Bassa Vah
Batk: Batak
Bhks: Bhaiksuki
Brah: Brahmi
Brai: Braille
Cakm: Chakma
Cari: Carian
Dogr: Dogra
Dupl: Duployan shorthand, Duployan stenography
Egyp: Egyptian hieroglyphs
Elba: Elbasan
Gong: Gunjala Gondi
Gonm: Masaram Gondi
Gran: Grantha
Hatr: Hatran
Hluw: Anatolian Hieroglyphs (Luwian Hieroglyphs, Hittite Hieroglyphs)
Hmng: Pahawh Hmong
Hung: Old Hungarian (Hungarian Runic)
Java: Javanese
Khoj: Khojki
Kthi: Kaithi
Lana: Tai Tham (Lanna)
Lina: Linear A
Lisu: Lisu (Fraser)
Mahj: Mahajani
Maka: Makasar
Mand: Mandaic, Mandaean
Mani: Manichaean
Marc: Marchen
Medf: Medefaidrin (Oberi Okaime, Oberi Ɔkaimɛ)
Mend: Mende Kikakui
Merc: Meroitic Cursive
Mero: Meroitic Hieroglyphs
Modi: Modi, Moḍī
Mroo: Mro, Mru
Mtei: Meitei Mayek (Meithei, Meetei)
Mult: Multani
Narb: Old North Arabian (Ancient North Arabian)
Nbat: Nabataean
Newa: Newa, Newar, Newari, Nepāla lipi
Nshu: Nüshu
Orkh: Old Turkic, Orkhon Runic
Osge: Osage
Palm: Palmyrene
Pauc: Pau Cin Hau
Perm: Old Permic
Phli: Inscriptional Pahlavi
Phlp: Psalter Pahlavi
Plrd: Miao (Pollard)
Prti: Inscriptional Parthian
Rohg: Hanifi Rohingya
Samr: Samaritan
Sarb: Old South Arabian
Sgnw: SignWriting
Shrd: Sharada, Śāradā
Sidd: Siddham, Siddhaṃ, Siddhamātṛkā
Sind: Khudawadi, Sindhi
Sogd: Sogdian
Sogo: Old Sogdian
Sora: Sora Sompeng
Soyo: Soyombo
Takr: Takri, Ṭākrī, Ṭāṅkrī
Tang: Tangut
Tavt: Tai Viet
Telu: Telugu
Thai: Thai
Tirh: Tirhuta
Wara: Warang Citi (Varang Kshiti)
Xsux: Cuneiform, Sumero-Akkadian
Zanb: Zanabazar Square (Zanabazarin Dörböljin Useg, Xewtee Dörböljin Bicig, Horizontal Square Script)