The Safari Zone is there, and it functions.
TERU-SAMA is a number of items, with only 2 having any functionality at all. 06h is the Town Map from Pokemon Red and Blue, with only small functionality existing. 38h works as the Poke Flute. The TERU-SAMA were eventually reused to make new items in Pokemon Crystal, such as the GS Ball.
"If they got rid of the beta files on the cartridge, could they have had space to expand kanto? Like, to add a safari zone, the cinnabar island, pewter museum, viridian forest..."
The Safari Zone, does in fact, exist in Pokemon Gold. It's got unused, but fully functional warps, in Fuchsia City. Same place as before and all. It has a hallway as well, with no attendant or person in sight. You can enter the Safari Zone from here, but can't get out. I have personally managed to make this functional before (see signature).
Linking in with the Safari Zone, there are 2 other tables of Pokemon that can run away. They are as follows:
Table 1 (25% chance to flee a turn):
Eevee
Porygon
Togetic
Umbreon
Table 2 (50% chance to flee a turn):
Articuno
Zapdos
Moltres
Table 3 contains the Legendary Beasts, who have a 100% chance to flee. I believe that Table 1 could have been the start of the Safari Zone, but I am not entirely sure. One oddity here is that Espeon doesn't flee, as well as Baby Pokemon Evolutions.
Unused text relating to Pokemon eating and getting angry in the Safari Zone also exist. Eating starts at 10168A, and ends at 101695. Angry starts at 1016A1, and ends at 1016AB. In PokeText, this can be viewed in "Others 1" at Blocks 144 and 146.
As for Cinnabar Island, it is possible to restore it. It's just the Map Size that takes up space, not whatever space there is. With the space the maps have, you can restore Cinnabar. In fact, an unused palette for the Gym exists.
Pewter Museum isn't hard to restore either as long as you can make the Warps. To do this, you need to know the Bank and Map Number for it. Sadly, I don't know this myself.
Viridian Forest is a difficult one, but I don't think it would be that hard to do.
TCRF has more information here;
https://tcrf.net/Pokémon_Gold_and_Silver