How did you establish that she had "disappeared" by 1900?

Working from Free Cen, in 1891 there appear to have been 4 Elizabeth Haynes, born 1866/80 living in Plymouth/ Stoke Damerel, 2 single, 2 married. The one that attracted my attention was still in Devonport in 1901. The other one seems to have married in 1897

Of course without any additional detail anything further would bre totally speculative.