30 mm/ 1.18 Inch PTFE Magnetic Stirrer Mixer Stir Bar, White Color, Stir Plate Magnet (4 Pack)

Q: Can these be autoclaved?

I would have to assume so. They’re scientific instruments. Generally, with a few exceptions, there designed to be regularly sterilized. I regularly send mine through a pressure cooker at 15 PSI in liquid cultures

Q: Could these be used with a sensor for an alarm system (Simplisafe)? I have double hung windows so need a slim magnet to put opposite the sensor. Thx

Probably, but I suspect it would be much easier to use one of these that's the size you want: https://www.amazon.com/s?k=neodymium+disc+magnets&crid=3RBE9SKKGYW9W&sprefix=Neodyn%2Caps%2C235&ref=nb_sb_ss_ts-doa-p_5_6

Q: is there any possibilities that these could degrade and leak into the water? any easy way to see it? thanks

There is nothing to leak. Inside is a solid magnet. It's possible that after using them a long time, friction could wear out the plastic coating, but that will probably take years and you'll see it slowly degrading. Just take a quick glance before you use them and if they look like they are in good condition, they are.