It stands for allowing someone to continue doing or use something that is normally no longer permitted (due to changing regulations, internal rules etc.)

It means to give benefits to old users that you have stopped giving to new users. For instance if your online service is completely free and then you restrict certain features for new users, you might grandfather old users into the premium plan to keep them happy (since it was free for them initially)

I am a grandfather and I am involved already more than 35 years in the technologies' developments. I studied electronic lamps and now I am involved in projects with wifi, lifi, Bluetooth controls. I know how it went, I witnessed it. Youngsters are coming now and think, that the world is just created. Sadly for them, it is not. It is enough mature for grandfathers to be in.
all the best
Val

Before we investigate the question further, let us understand the use of Grandfather Clause. A grandfather clause is an exemption that allows persons or entities to continue with activities or operations that were approved before the implementation of new rules, regulations, or laws. A grandfather clause only exempts people or entities engaged in specified activities before new rules being put in place, while all other parties must abide by the new rules. However, these clauses effectively place two sets of rules or regulations on otherwise similar businesses or circumstances, which can create unfair competitive advantages for grandfathered parties. In these situations, grandfather clauses may only be granted for a set period.
The origin of the term refers to statutes put in place after the Civil War by seven Southern states in an attempt to block African Americans from voting while exempting white voters from taking literacy tests and paying poll taxes required to vote. In the statutes, white voters whose grandfathers had voted before the end of the Civil War were exempt from taking the tests and paying the taxes under the grandfather clause. The statute was deemed to be unconstitutional by the Supreme Court in 1915 because it violated equal voting rights, but the use of the term indicating rights prior to rule changes carries on.
Depending on specific circumstances, grandfather clauses can be implemented in perpetuity, for a specified amount of time, or with specific limitations. In situations where this clause creates a competitive advantage for the grandfathered party, exemptions are usually granted for a specified period to allow existing businesses to make the changes necessary to comply with new rules and regulations. Clauses with specific limitations may also be put in place to prevent unfair competition, such as prohibitions on the expansion, remodelling, or retooling of an existing facility.
The term is actually not ‘grandfather you in’ but ‘grandfather in’ which means, to exempt certain people or businesses from new limitations or restrictions, thus allowing them to continue doing or benefiting from something as they did before. This can be done through the use of a "grandfather clause." A noun or pronoun can be used between "grandfather" and "in." but grandfather someone or something in means to protect someone or a right using a grandfather clause. Grandfather clause is discussed at the beginning of the answer.
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In the tech world, when someone says they want to "grandfather you in," it means they want to include you in a new policy, program, or opportunity, even though you may not meet the usual requirements or criteria. It's like giving you a special exception or privilege to be part of something based on your past involvement or relationship.

To "grandfather you in" typically means allowing someone to continue benefiting from certain privileges, rights, or conditions that they already have, even though new rules or requirements have been established that would otherwise exclude them. This phrase originates from the idea of a grandfather clause, which was a legal provision that exempted certain people or entities from new regulations or restrictions if they had already been doing something before the new rules were enacted. So, when someone says they will "grandfather you in," they are essentially allowing you to maintain your current status or benefits despite changes that might affect others.