What words are used in Spanish to mean "pantyliner"?
Although there is no Spanish entry for "pantyliner" in wikipedia (your linked source), there is one in Wordreference, which translates as "pantiprotector" and "salvaslips". None are in the DRAE. I think I have never heard of "pantiprotector" but may have heard of "salvaslips" (or even "salvaslip").
Nevertheless, I believe that the more widespread term used for "an absorbent piece of material used for feminine hygiene [...] worn in the gusset of a woman's panties [...] for daily vaginal discharge, light menstrual flow, tampon and menstrual cup backup, spotting, post-intercourse discharge, and urinary incontinence" is "compresa".
The term "compresa" can refer also to some sort of sanitary pad, but it also refers to the "compresa higiénica", which is
- f. Tira desechable de celulosa u otra materia similar que sirve para absorber el flujo menstrual de la mujer.
"Compresa" (pad) is a more general term, but normally used to mean the thicker ones.
Salvaslips are thinner and more discreet, but cannot absorb as much. Because of that, they are used only in the last days of your period, or in the days before as a protective measure against an unexpected start of it. Hence the name "salva slip" or "protege slip": they save/protect your panties from stains and give you some time to find a proper pad or tampon.
It is not in the dictionary (yet), but every woman will understand what you mean.