I am sure this question emerges frequently, but I have seen the same questions with different circumstances. Here's mine, assuming my previous employer is "A" and my new employer is "B".

I have worked at "A"(a pathetic organisation with no management, miscommunication, etc) for 2 years. I have now been selected for "B" and got a raise of 70%, they didn't ask for a relieving/experience letter and were open to not requiring it in future too. At "A" there was the usual 3-month notice period if I wanted to leave otherwise a buyout option was available where I could leave immediately with 3 months' salary to be compensated for getting a relieving/ experience letter. I have heard from my friend that the salary compensation would be based on Basic salary but they have calculated my overall salary to be paid.
I got an email from my previous Hr where she just mentioned the amount directly without showing the calculation deductions/additions or any tabular separation of the amount.
Now that "B" is very open and didn’t ask for any letter, Although they will be doing BG verifications which I notified about all these conundrums with "A", the question arises whether relieving/experience is necessary. I know It is recommended to obtain one, but I wanted to know how it will affect my career in future If I refuse to obtain one. My current HR informally stated that the past practices are being in a changing phase and these formalities should be delivered to the employee after they resign immediately. Also what are the corporate laws suggests in this situation. I also doubt whether “A” will give my letters even after paying them the lieu amount. Thanks for your help in advance.