This is the era of dynamic data models with Cassandra, DynamoDB, and other products that specialize in just-in-time data structures. Even "traditional" SQL database vendors like Microsoft, PostgreSQL, and Oracle let you store JSON, XML, and other loosely- or even non-structured data. Come to think of it, we've always been able to add columns to database tables, and we've certainly been able to model the relationship between something like a form and a collection of fields inside a form.
But if I create a custom item type in LastPass with four fields in it and want to add a fifth one? I have to create the new item type from scratch and copy the existing data over to the new item type manually. If we can do it manually, you can do it programmatically. And I suspect it will be easier programmatically unless it's a mess in there: you should be able to add another field to the collection of fields in this item type, just like if we were, you know, using the item designer.
For a product that prides itself on usability, this sure feels Neanderthal. Please fix this already.