Yes, this is one option for nursing mothers. Another is waiting for a change in mucus - I tend to always have it, but it changes to fertile-looking shortly before my cycles resume.
Another is charting temperature rather than relying on a mucus-only method.
I appreciate FrDavid96’s response.
“Contraceptive mentality” is a phrase I really wish would go away. Contraception is an act - it’s something you do to the sex act or to yourself/your spouse to prevent sex from naturally occurring. In your example, you’re not doing anything to the sex act. You’re just doing something commonly done anyway for hygienic or comfort reasons that minimally impacts chances of conception, if it impacts it at all. That’s different than a method like withdrawal (which is also not reliable), but actually prevents the sex act from being completed.
Simply not wanting a baby is not a sin. Couples should be open to any children that God sees fit to bless them with, and that’s a requirement of valid marriage, but not actively desiring a child is not, by itself, sinful.