I also use sentiment analysis quite regularly. But there are some quirks you need to be aware of. E.g.
Classify["Sentiment", "The weather is horrible."]
gives "Negative" as expected.
Classify["Sentiment", "The weather is horrible!"]
gives "Positive". The exclamation mark has an interesting effect on nearly any statement:
Classify["Sentiment", "This day is the worst of my life and I am so sad!"]
is classified as "Positive". In many situations these issues can be mended though.
I think that there might be larger issues to consider than "only" a gradual change of language over time. Also, when analysing for example, social media posts in real time during political debates, one would use rather different Classifiers for say the US and the UK.
I would be quite interested in the analysis of Bertrand Russell's letters and would love to see the results.
Cheers from Scotland,
Marco
PS: you can easily build your own classifier of course, using datasets like:
https://www.baeldung.com/cs/sentiment-analysis-training-data
https://analyticsindiamag.com/10-popular-datasets-for-sentiment-analysis/
https://blog.cambridgespark.com/50-free-machine-learning-datasets-sentiment-analysis-b9388f79c124
https://www.kaggle.com/c/sentiment-analysis-evaluation/data
https://www.kaggle.com/kazanova/sentiment140
This might be interesting, too:
https://epjdatascience.springeropen.com/articles/10.1140/epjds/s13688-017-0121-9
It is also possible to use data from GDELT, which has newspaper articles and has data on how positive/negative the articles are. Not ideal for sentence by sentence analysis.
For a lecture I have "redone" the analysis here:
https://www.jstor.org/stable/118328
based on the automatic classification in the Wolfram Language.