Hi,
Most certainly the punishment carried on well into the 1900's, I know that my grandfather was punished for speaking Welsh.
My grandfather was totally Welsh speaking (didn't know a word of English as a young child) he used to tell me he was punished in school for speaking his own tongue, hit with a rule and made to stand in a corner( this would be in the early 1900's in school, he was born in 1898). He learned his English at school. This was in South Wales (Aberdare).
The 1911 census for him says Welsh (as with his parents and siblings) - I assume he must have knew a little English at the time , but he was not that fluent in it and only had to speak it in school, so the family didn't write both - Even as an old man when I knew him he had trouble with the English Language , he could speak it ok, but he'd begin a sentence in English to me and slip straight into Welsh without him realising it. ( He had to think of the words before he spoke in English , where his thoughts were in Welsh - so his words would slip out in Welsh ).
My husband's dad who today only speaks English started life only speaking Welsh. Up to the age of about 6 he only could speak Welsh - Then his dad went away to war. When he came back ( the war changed his father - screwed up, usual story for some of them , drink, temper you name it) he absolutely forbid his children to speak any Welsh (and I mean forbid!!!), don't ask me why ( perhaps something happened in to him in the army). My father-in-law still won't speak Welsh , he pretends he has forgotten it all , but I do not believe him -you can't be fluent in one language only (even though he was just a young boy) and then totally forget it all- He knows it alright ,you've only got to swear in Welsh and he grins.
I blame the government of my childhood for myself . My Mother ( who is Irish,) tried her very best to send me to our local Welsh (as in language) infant and primary school as she wanted me to be taught in my country's mother tongue (My dad's Welsh) - but the policy of the schools in those days (in the 1970's) was at least one parent had to be fluent in the language, my dad is not fluent enough . I can imagine my mum strolling up to the school and giving them her best with an Irish accent lol.
This has apparently has all changed now, my husbands cousin sends her 2 boys to a Welsh school, neither her or her husband can speak one iota of Welsh. One of the boy's is doing really, really well at the school ,top grades in all subjects - but the other is struggling to the verge of pulling him out .
The Welsh that I do have has come from all over the place (life) and also from my compulsory Welsh lessons in secondary education .( Schools in Wales had compulsory Welsh language lessons during the 80's for the first few years in secondary. I don't know when it first came in, or even if it's still compulsory today)
Kind Regards