If you have formatted the cell to fraction then just typing ~~ ~~`=8/16`

`0 8/16`

(acknowledgement to erAck) will enter the figure. Note that LO will convert it to its simplest form, that is 1/2.

2021-05-03 As suggested by @Vanadium and reinforced by @erAck , choose a denominator of 16. Format Cell as Fraction `# ??/16`

to get compound fraction with 16ths.

~~ If you want 8/16 to show then you will have to format the cell to ~~*Text* and then just type *8/16*. You will not be able to use this in calculations.

Date recognition is wanted by most people and is strongly integrated into Calc. There may be other workarounds but before spending too much time on looking at ways to get around it I would read How do you disable date formatting in LibreOffice Calc?

I would consider carefully before using fractions, except where you are teaching them specifically, as they are not used by many people and unfamiliarity might have unintended consequences for the reader. Cheers, Al

Entering Fractions