What will happen if didn't delete flash player after 31 December 2020 on MacBook?

@defaultkr72k80yjs4u  Sorry I meant Standalone SWF player. You can find the Adobe one here if you dont want to hunt down an off brand one https://www.adobe.com/support/flashplayer/debug_downloads.html  Get the file with Flash Player Projector as its name. Then play any swfs you have saved with it.

As for sites the ones that care and were not fooled into thinking things are hopeless are working on solutions. For the ones that dont seem to be working on a solution talk to them I guess. Let them know about Shumway, ruffle.rs, waflash, cheerpX, and so on. There are people working hard to make flash better than ever. Sometimes telling people that there are solutions albeit currently imperfect ones that are being improved is all it takes.

Ive found SuperNova SWF Enabler to be helpful in viewing flash online.

You can also help in the preservation of swf files through using and telling others about https://bluemaxima.org/flashpoint/

What can I use if flash player couldn't use in 2021? Any other software that you recommend that I could use? Thank you so much!

Nothing bad will happen. Flash Player will stop working, that's all. Chances are you'll never miss it. 

See the official announcement below.

https://www.adobe.com/products/flashplayer/end-of-life.html

It will simply stop working in the browser while staying on your computer.

There are standalone players that you can find through a web search. Just look up standalone swy or flash player. Get a good one. Take a swf file you have saved and load it with the player. If you want sites to play flash files in a browser after 2020 tell them about ruffle.rs by looking for their contact page and talking to them.

https://community.adobe.com/t5/flash-player/psa-playing-flash-after-2020/td-p/11716613