Had to do this recently for a lot of test projects that were created in our Jira sandbox instance.

It’s a small python script using Jira API and the requests python library.

import requests
from requests.auth import HTTPBasicAuth

domain = "https://MY_JIRA_INSTANCE.atlassian.net"
admin_user = "JIRA_ADMIN@MY_COMPANY.com"
admin_token = "JIRA_ADMIN_TOKEN"
project_query = "PROJECTS_I_WANT_TO_DELETE"

def getJson(url, auth):
    r = requests.request(
        "GET",
        url,
        headers={"Accept": "application/json"},
        auth=auth
    )
    return r.json()

def deleteProject(id, auth):
    r = requests.request(
        "DELETE",
        domain + "/rest/api/3/project/" + id,
        auth=auth
    )
    return r.text

search_url = domain + "/rest/api/3/project/search?query=" + project_query
auth = HTTPBasicAuth(admin_user, admin_token)
json = getJson(search_url, auth)

projectIds = []

# append results across all pages, while there's still a nextPage, to projectIds array
while "nextPage" in json:
    nextUrl = json["nextPage"]
    for searchResult in json['values']:
        # optional safety check to make sure the right project is being added to the deletion array
        # if "PROJECT_NAME_I_INTEND_TO_DELETE" in searchResult["name"]:
        projectIds.append(searchResult['id'])
    print("Number of project IDs found matching the search query: " + str(len(projectIds)))
    json = getJson(nextUrl, auth)

# append a single page, or the last page of results, to projectIds array
for searchResult in json['values']:
    # optional safety check to make sure the right project is being added to the deletion array
    # if "PROJECT_NAME_I_INTEND_TO_DELETE" in searchResult["name"]:
    projectIds.append(searchResult['id'])
print("Number of project IDs found matching the search query: " + str(len(projectIds)))

# delete projects in projectIds array
for index, id in enumerate(projectIds):
    print("Deleting project " + id + ". Projects remaining: " + str(len(projectIds)-index))
    print(deleteProject(id, auth))