Since 1.13 the text strike-through has been lowered by 1px downward when compared to version 1.12.2 and prior. This is most apparent when using a strike-through with the hyphen or the en/em dashes due to it (previously) creating a seamless hortizontal line.

/tellraw @a {"text":"——————————","strikethrough":true}

Current as of 1.21:

How it used to look:

Side by side comparison (normal font (1.12.2/1.13 pre5) vs 1.13 (pre7+)):

Side by side comparison (unicode, 1.13 pre5 vs 1.13 pre8)