A Regex to remove digits except for words starting with #

How about capturing the wanted and replacing the unwanted with empty (non captured).

gsub("(#\\S+)|\\d+","\\1",x)

See demo at regex101 or R demo at tio.run (I have no experience with R)

My Answer is assuming, that there is always whitespace between #foo bar #baz2. If you have something like #foo1,bar2:#baz3 4, use \w (word character) instead of \S (non whitespace).

You could split the string on spaces, remove digits from tokens if they don't start with '#' and paste back:

x <- "table9 dolv5e #10n #dec10 #nov8e 23 hello"
y <- unlist(strsplit(x, ' '))
paste(ifelse(startsWith(y, '#'), y, sub('\\d+', '', y)), collapse = ' ')
# output 
[1] "table dolve #10n #dec10 #nov8e  hello"