In a very similar fashion as its predecessor and successor (Underground 1 and Most Wanted respectively), some Underground 2 mods will eventually go into obscurity, and most of them are offline, usually due to inclusion of NFS-CfgInstaller, which Antivirus Softwares claim it as virus (which is actually a false positive as this is harmless tool). Again, if the car has been found after being lost since those years, the text will be crossed out like this.
Because there are creators who are not willing to reupload most if not all of their countless cars (if they made many of them), I decided to split this into two categories: specific lost cars, and specific authors/creators whose most or all of their creations are lost.
Specific cars that are yet to be rehosted or lost completely:
Dodge Charger by maxXR (https://web.archive.org/web/20150416201103/http://www.nfscars.net/underground2/file/14515/): showcased by RASTAKITTEN of U4G. (https://www.youtube.com/watch?v=14fqcQ5DRGA) Currently, you can't find this specific version of the mod on NFSCars anymore. As far as I know, it replaces Lexus IS300 and had some kind of customization (including vinyls, with some bugged UV mapping). Until June 18, 2023, I never knew that the author was maxXR before visiting RASTA's video via Internet Archive's Wayback Machine (I linked the archived link to it yet you can't download anything yet). Not to mention about the fact that this one is the rarest '69 Charger for Underground 2, as for most of the time you will find RuubW's Charger instead.
Specific authors whose some, most, or all of their mods are offline and yet to be rehosted:
RuubW: the well known mod creator known for making various mods converted from later NFS games, ranging from Most Wanted to Undercover. Only few mods exist online, because they are meant to upload all of their mods to NFSCars only, and thanks to NFSCars site redesign, most of their mods are gone for the reason I explained earlier.
SPYKE: another well known mod creator, they are known for making cars converted from Forza (back when Riva's rips existed). Usually they are prone to crashing due to high texture resolution (before NFSU2 Unlimiter kicked in). All of their mods are gone for good from NFSCars. I don't remember where those cars are rehosted.
cobra0281: some or most of their cars are completely gone from NFSCars. Included for the sake of completeness.
kas_87: same thing as cobra0281 (still wish to get kas' Subaru Impreza 555 from DiRT 1 for NFSU2).
I will post these ones, since these ones are not published on any sites (Made by GM25):
1) Porsche 908-04 Concept: https://www.mediafire.com/file/5fwwvy839qgwm92/908_Concept.rar/file
2) Porsche 911 (964) Carrera RS N/GT: https://www.mediafire.com/file/dr7c3na41r78bcg/911_%2528964%2529_Carrera_RS_NGT.rar/file
3) Toyota S-FR: https://www.mediafire.com/file/fhbxvqxtwuvyfy2/S-FR.rar/file
4) FSO Syrena Sport: https://www.mediafire.com/file/yw2u303blxymcju/Sport.rar/file
5) Peugeot 205 GTI: https://www.mediafire.com/file/7r0xgx4b1mzah5e/205_GTi.rar/file
6) Reliant Robin: https://www.mediafire.com/file/feb7mcul66a4a2a/Robin.rar/file
7) Renault Laguna Coupe (From NFSCars): https://www.mediafire.com/file/45ealywa0ftwqni/Laguna_Coup%25C3%25A9.rar/file
Deleted / Unavailable car mods from NFSCars:
Renault Twingo Mk3 (by kas_87): https://www.nfscars.net/need-for-speed-underground-2/6/files/view/8652/
Citroen Saxo S1600 (by kas_87): Google nfs ug2 citroen saxo
Bumped intentionally because I found the name of the author of lost '69 Charger mod (the one that replaces IS300). The author itself (maxXR) is still offline and so the car I mentioned (their other two cars, Freightliner and Kamaz, are still up on NFSCars)
Found an interesting site that reuploaded some of the old vehicles for NFSU2 (some links like the one of Scion tC redirect you to the wrong car):
http://www.modgaming.ru/nfs-underground-2/nfsu2-auto/
Caution: contains repackaged zip files without readme files and sometimes incorrect author names.