Reread what I wrote. Plenty of WWII games have similar tanks and it doesn't cause a problem. The T-26 would be available earlier than T-70 (also slower) and would create variation for starting strategies.
New units have been added to the game when people thought no more should be added; those units didn't break the game. The gameplay as of now is not entirely satisfactorily and it's not due to new units but other unrelated stuff *cough*British turtle faction*cough*.
And if by your argument, i doubt anyone would support the release of a T-26 tank for the soviet faction, let alone seriously consider it.
I wouldn't mind seeing it put in for historical flavor but have you considered how you would actually balance it with any ideas apart from a mere suggestion? Like Cost in MP and Fuel, whether it takes the role of AI or AT and its scalability? Will it repeat what happened with the old Soviet Industry T-70 rush or will be it extremely unviable.
We also have to consider various other units that are still
unviable or very niche roles such as the USF WC51 mit 50cal and M8 Greyhound. Will the T-26 become OP at launch and then nerfed into submission? You can't just say "oh these tanks were historically important" and then provide nothing else.
Other users have also mentioned that especially the T-26 is outdated garbage during the War and especially the multiplayer time frame which takes place 1943-1945. The Panzer III though is perfectly good exception, except we have no idea why Relic never implemented it.
Overall I don't think most people would want a T-26, since if you describe it as support, wouldn't it die instantly to any form of AT? and what other tanks will it face in the early game? 222 Scout cars and Luchs? The T-70 easily supplants that role for it and it would again just be a waste of resources. Think these ideas through.