How do you remove old adhesive left on bathtub bottom from decals?

Vinegar to the rescue:

https://homeguides.sfgate.com/adhesive-strips-off-bathtubs-28645.html

Also, WD40 will take it off.

Here are 4 ways to try:

https://www.wikihow.com/Remove-Bathtub-Decals

Hello. You might try saturating them with some Goo Gone. Then hopefully the adhesive seal may be broken. You may then be able to peel/pull them off. Use something plastic like a credit card or depleated gift card as a flexible spatula and scrape them off. Goo Gone or WD40 should remove the adhesive residue.

WD-40 will work. It is quite smelly but if you place it on the strips and then cover with plastic it will do its work. Use a soft plastic scraper to remove as much as possible after using the WD-40 soak. It may take more than one attempt to get all the adhesive off. Then you can scrub the bathtub well using Dawn dishwashing liquid and a soft scrub brush.

here is info to help you

https://www.youtube.com/watch?v=Z3yVHWlnEms

WD40 or GOO GONE. You can try acetone nail polish remover

Hi there! Lots of good ideas here...with old adhesive, sometimes you just have to try a combination of things until you finally get the last of it. It can be very stubborn!

In this post, I removed some very old, dried-on adhesive from vintage lockers. I didn't want to destroy the original paint finish, and I'm sure you're thinking about that with your tub finish, too. Hopefully some of the tricks I used will help you!

Acetone, the stuff you buy in the hardware store not nail polish remover. It is way stronger. Be sure to open a window when using or turn on exhaust fan.

The simplest adhesive remove is a paste of cooking oil and baking soda. Add any oil to baking soda and make a paste you apply and rub with your fingers. Keep rubbing until the adhesive is dissolved. Clean with dish soap to remove the oil.

If it is residue, you can use vegetable oil to remove it. However, once you get it off there will most likely be a bit of fading on the tub where the decals were not so you will still be able to see where they were placed.

Use goo gone to remove it that will work you could try to use a streight edge razor but keep it flat so not to scratch

There are many good adhesive removers on the market. Here is just a few.

https://www.bobvila.com/articles/best-adhesive-remover/

wd40

WD - 40 nor maybe Surgical Spirit!

You could try a hairdryer and a plastic putty knife. Otherwise yes, most things are smelly like WD40 or Goo Gone but those will definitely work.