Hi Dani,
Yes, even if a Quote is in Active Status, we can access the Products/Quote Lines and Edit the Products fields by default, which is a Dynamics 365 default behavior.
If you want to make form of related quote line is read-only when a Quote is in Active Status, you can use javascript code to lock quote line form based on status of the quote.
js code:
function lockQuoteLine(executionContext) {
let formContext = executionContext.getFormContext();
var QuoteTD = formContext.getAttribute("quoteid").getValue()[0].id;//quote lookup id
//get status of quote
Xrm.WebApi.online.retrieveRecord("quote", QuoteTD, "?$select=statecode").then(
function success(result) {
var statecode = result["statecode"];//value
var statecode_formatted = result["statecode@OData.Community.Display.V1.FormattedValue"];//text
if (statecode == 1)//status is active
{
//lock all fields of the form
formContext.data.entity.attributes.forEach(function (attribute, index) {
let control = formContext.getControl(attribute.getName());
if (control) {
control.setDisabled(true)
}
});
};
},
function (error) {
Xrm.Utility.alertDialog(error.message);
}
);
}
Go Settings > Customizations > Customize the system > Web Resource to add the js code.

Then Expand Entities to fins quote line entity, open the main form.

Open form properties dialog to add the js resource:

Save and Publish all customizations.

