Yes, the old Proto-Indo-European locative case did disappear into the ablative in Latin. However, the names of cities, towns, and small islands and the words domus, humus, and rū s held onto those antique forms.

As for those special words, they have special locative forms:

domus becomes domī (at home)

humus becomes humī (on the ground)

rū s becomes rū rī (in the country)

As for the names of cities, towns, and small islands, here are the locative forms:

Table 8-5 Locative Case Across the Declensions

Locative case endings really don't offer much new for you to learn. Here's a shortcut: All the locative forms are the same as the ablative. Only first and second declension singular place names (like Roma) are different.

TABLE 8-6 is a chart that summarizes everything in this section:

Table 8-6 Place Constructions