MySQL:

ST_POINTN

ST_PointN(ls, N)

Returns the N-th Point in the Linestring value ls. Points are numbered
beginning with 1.

ST_PointN() handles its arguments as described in the introduction to
this section.

URL: https://dev.mysql.com/doc/refman/8.0/en/gis-linestring-property-functions.html

Example

mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT ST_AsText(ST_PointN(ST_GeomFromText(@ls),2));
+----------------------------------------------+
| ST_AsText(ST_PointN(ST_GeomFromText(@ls),2)) |
+----------------------------------------------+
| POINT(2 2) |
+----------------------------------------------+