The biggest one in the EU being the "No KID, no deal" (for funds, derivatives, etc.): no Key Information Document, then cannot be offered to retail.
That is unless retail is an accredited investor (say a letter from a bank saying you have more than 1m EUR in investments): then it can trade/invest in everything.
Or for example if you own a company (say private weath management) that's non-custodian (say you manage your wealth using your own company, and shares of your company are privately owned) and you're considered a professional in the finance world, you can ask your broker to let you trade / invest in basically every single product under the sun.
AFAIK pre-IPO stocks are just as vanilla as regular stocks and do not need the KID notice.
In addition to that Anthropic (in the EU), says you can enter pre-IPO using Revolut, Trade Republic, eToro or using bigger players like Saxo or IBKR.
Dunno why GP doesn't have access to it in Poland.
I confess I’m about 17 years out of practice in the Polish market, and about 12 years out of touch with how the European industry shakes out on the whole, so I’m not entirely familiar with the legal or business structures.