Season 10 is the cut-off for most free streaming libraries. It represents the end of the "early era" before the final two seasons (which are usually locked behind paywalls).
The show frequently airs in syndication on networks like TBS in the US or E4 in the UK. You can often use your cable provider login to stream episodes on their respective apps. Breaking Down the First 10 Seasons the+big+bang+theory+season+1+2+3+4+5+6+7+8+9+10+free
These free, ad-supported streaming services often have dedicated sitcom channels that broadcast episodes 24/7. Season 10 is the cut-off for most free streaming libraries
Here's a brief summary of each season: