If you wanted a somewhat cruder approach using basically ubiquitous tools, you could do something like this:

$ grep '^ *-' /tmp/foo.txt | grep -v ': *$' | sed 's/ *- //' | tr '\n' ',' | sed s'/,$/\n/'
Harry potter,Perfect Blue,Jurassic world,Jurassic Park,Jedi,Star wars,The clone wars,MCU,Gumball ,Flapjack,Steven Universe,Stars vs. the forces of Evil,Wordgril,Flapjack 

Here I'm first using grep '^ *-' to get all lines starting with any amount of whitespace and a leading dash, then piping that to grep -v ': *$' to remove anything with a colon at the end (including those with whitespace after the colon), then using tr '\n' ',' to replace all newlines with commas, and then sed s'/,$/\n/' to replace the trailing comma with a newline again (although sed is finicky across platforms wrt newlines, so you may want to just replace it with an empty string instead).

The above is hardly an efficient approach, but it does the job.

If you can't install a dedicated tool like yq but don't mind creating a standalone script, python would be able to do this out of the box on pretty much any computer, calculator or toaster you can get your hands on in 2026:

#! /usr/bin/env python3

import yaml
import sys

def parse_yaml(filename):
    with open(filename) as fd:
        return yaml.safe_load(fd)

def get_leaf_nodes(data_iterable):
    output = []
    for v in data_iterable:
        if isinstance(v, dict):
            output += get_leaf_nodes(v.values())
        elif isinstance(v, list):
            output += get_leaf_nodes(v)
        else:
            output.append(v)
    return output

print(",".join(get_leaf_nodes(parse_yaml(sys.argv[1]))))

This takes the first argument on the command line, parses it as yaml, finds all leaf nodes recursively, and prints a comma-separated list of the results.

If you can stick to valid YAML like your example is, you can use a reasonably short yq command to get a comma-separated string of all scalar values:

$ yq -r '[.. | scalars] | join(",")' /tmp/foo.txt                
Harry potter,Perfect Blue,Jurassic world,Jurassic Park,Jedi,Star wars,The clone wars,MCU,Gumball,Flapjack,Steven Universe,Stars vs. the forces of Evil,Wordgril,Flapjack

.. goes down the tree recursively, scalars filters out only scalar values, [] around those two makes them an array, and piping it all to join(",") makes it into a comma-separated string.

This is technically yaml I think, a list (with one entry) of lists that contains mostly single items but also one other list. You should be able to parse this with a yaml parser like pythons built in one.

Note that yaml is picky abiut the syntax though, so it wouldn't be able to handle deviations.